Light-emitting element and light-emitting device

ABSTRACT

It is an object of the present invention to provide a light-emitting element having a layer containing a light-emitting material and a transparent conductive film between a pair of electrodes, in which electric erosion of the transparent conductive film and metal can be prevented, and also to provide a light-emitting device using the light-emitting element. According to one feature of the invention, a light-emitting element includes a first layer  102  containing a light-emitting material, a second layer  103  containing a material having a donor level, a third layer  104  including a transparent conductive film, and a fourth layer  105  containing a hole-transporting medium between a first electrode  101  and a second electrode  106 , in which the first layer  102 , the second layer  103 , the third layer  104 , the fourth layer  105 , and the second electrode  106  are provided sequentially, in which the second electrode  106  has a layer containing metal.

This application is a continuation of copending U.S. application Ser.No. 14/528,209, filed on Oct. 30, 2014 which is a continuation of U.S.application Ser. No. 13/155,693, filed on Jun. 8, 2011 (now U.S. Pat.No. 8,878,159 issued Nov. 4, 2014) which is a continuation of U.S.application Ser. No. 10/573,929, filed on Mar. 30, 2006 (now U.S. Pat.No. 7,964,864 issued Jun. 21, 2011) which is the U.S. national stage ofPCT/JP2005/018243 filed Sep. 27, 2005, which are all incorporated hereinby reference.

TECHNICAL FIELD

The present invention relates to a light-emitting element having a pairof electrodes and a layer containing an organic compound capable ofemitting light by applying an electric field thereto. The inventionfurther relates to a light-emitting device having such a light-emittingelement.

BACKGROUND ART

A light-emitting element using a light-emitting material has advantagesof thinness and lightweight, high-speed response, DC low-voltage drive,and the like and is expected to be applied to a next-generation flatpanel display. In addition, it is said that a light-emitting devicehaving light-emitting elements disposed in matrix is superior to aconventional liquid crystal display device in a wide viewing angle andhigh visibility.

A light-emitting element is said to have the following light-emissionmechanism: voltage is applied to a light-emitting layer sandwichedbetween a pair of electrodes, electrons injected from a second electrodeand holes injected from a first electrode are recombined in alight-emission center of the light-emitting layer to form molecularexcitons, and then light is emitted by releasing energy when themolecular exciton returns to the ground state. A singlet excited stateand a triplet excited state are each known as the excited state and thelight emission is considered possible via any one of the excited states.

In order to enhance the characteristic of such a light-emitting element,the improvement of the element structure, the development of thematerial, or the like is performed.

For example, a method in which an optical length L from a light-emittingportion to a reflective electrode is controlled by sandwiching ITObetween the light-emitting portion and a reflective metal is given as ameans for increasing the external quantum efficiency withoutdeteriorating the luminance by controlling the distance from thelight-emitting region to the reflective metal (see, for example,Reference 1: Japanese Patent Application Laid-Open No.: 2003-272855).

FIG. 2 schematically shows an element structure disclosed inReference 1. In this structure, a transparent electrode 201, alight-emitting portion 202, a transparent conductive film 203, and ametal electrode 204 are stacked. By adjusting the thickness of thetransparent conductive film 203, the optical length L from thelight-emitting portion to the metal electrode is optimized to increasethe external quantum efficiency.

However, according to the structure disclosed in Reference 1, since thetransparent conductive film 203 and the reflective metal (metalelectrode) 204 are in contact, there is a problem of erosion. Here, itis known that the erosion might occur due to the difference in theirself-potential or the like, and also the erosion is referred to aselectric erosion (see, for example, Reference 2: Japanese PatentApplication Laid-Open No.: 2003-89864). Reference 2 describes theself-potential measured using a sodium chloride solution of 3.5% (liquidtemperature of 27° C.) and using silver/silver-chloride as a referenceelectrode. In the case of such a measurement, the self-potential ofaluminum known as metal having high reflectivity is approximately −1550mV, while that of ITO serving as a transparent conductive film (In₂O₃containing SnO₂ by 10 wt %) is approximately −1000 mV. Thus, thedifference between these self-potentials of aluminum and ITO is large.Therefore, when aluminum and ITO are stacked to be in contact with eachother, it is very likely that oxidation-reduction reaction progresses ata stacked interface between aluminum and ITO, which highly results inelectric erosion. Such a problem of electric erosion is generatedregardless of the combination of ITO and aluminum.

The self-potential is potential of a reaction to a reference electrodewhen the reaction is soaked in a certain solution in such a state thatcurrent is not applied from outside, that is, potential in a closed loopand is also referred to as resting potential.

DISCLOSURE OF INVENTION

In view of the above problems, it is an object of the present inventionto provide a light-emitting element having a layer containing alight-emitting material and a transparent conductive film between a pairof electrodes, in which electric erosion of the transparent conductivefilm and metal can be prevented. Moreover, it is an object of theinvention to provide a light-emitting device using the light-emittingelement.

According to one structure of a light-emitting element according to theinvention to solve the above problems, the light-emitting elementincludes a first layer containing a light-emitting material, a secondlayer containing an organic compound and an electron-supplying material,a third layer including a transparent conductive film, and a fourthlayer containing a hole-transporting medium between a first electrodeand a second electrode, in which the first layer containing alight-emitting material, the second layer containing an organic compoundand an electron-supplying material, the third layer including atransparent conductive film, the fourth layer containing ahole-transporting medium, and the second electrode are providedsequentially, in which the second electrode has a layer containingmetal.

According to another structure of a light-emitting element according tothe invention to solve the above problems; the light-emitting elementincludes a first layer containing a light-emitting material, a secondlayer containing an organic compound and an electron-supplying material,a third layer including a transparent conductive film, and a fourthlayer containing a hole-transporting medium between a first electrodeand a second electrode including metal, in which the first layercontaining a light-emitting material, the second layer containing anorganic compound and an electron-supplying material, the third layerincluding a transparent conductive film, the fourth layer containing ahole-transporting medium, and the second electrode are providedsequentially.

According to another structure of a light-emitting element according tothe invention to solve the above problems, a light-emitting device usingthe light-emitting element includes a first layer containing alight-emitting material, a second layer containing a material having adonor level, a third layer including a transparent conductive film, anda fourth layer containing a hole-transporting medium between a firstelectrode and a second electrode, in which the first layer containing alight-emitting material, the second layer containing a material having adonor level, the third layer including a transparent conductive film,the fourth layer containing a hole-transporting medium, and the secondelectrode are provided sequentially, in which the second electrode has alayer containing metal.

According to another structure of a light-emitting element according tothe invention to solve the above problems, the light-emitting elementincludes a first layer containing a light-emitting material, a secondlayer containing a material having a donor level, a third layerincluding a transparent conductive film, and a fourth layer containing ahole-transporting medium between a first electrode and a secondelectrode including metal, in which the first layer containing alight-emitting material, the second layer containing a material having adonor level, the third layer including a transparent conductive film,the fourth layer containing a hole-transporting medium, and the secondelectrode are provided sequentially.

In the above structure, each of the second layer containing an organiccompound and an electron-supplying material and the fourth layercontaining a hole-transporting medium may be formed in either asingle-layer structure or a multilayer structure where a plurality oflayers is stacked. Here, the organic compound is preferably an organiccompound having electron transportability and, particularly a metalcomplex having a ligand including a π-conjugated skeleton. Theelectron-supplying material is preferably alkaline metal, alkaline earthmetal, or rare-earth metal.

In the above structure, the second electrode may be formed withreflective metal in a single-layer structure or may be formed bystacking reflective metal and another electrode material.

With the structure of the invention, metal and a transparent conductivefilm are not in direct contact; therefore, the electric erosion due tothe difference in the self-potentials or the like can be prevented.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIG. 1 is a diagram explaining an element structure of a light-emittingelement according to a certain aspect of the present invention;

FIG. 2 is a diagram explaining an element structure of a conventionallight-emitting element;

FIG. 3 is a diagram explaining an element structure of a light-emittingelement according to a certain aspect of the present invention;

FIG. 4 is a diagram explaining an element structure of a light-emittingelement according to a certain aspect of the present invention;

FIGS. 5A and 5B are views each explaining a light-emitting device;

FIGS. 6A to 6E are views each explaining an electronic device;

FIGS. 7A and 7B are views each explaining a module mounted alight-emitting device;

FIG. 8 is a view explaining an electronic device;

FIG. 9 is a graph of a current density-luminance characteristic of alight-emitting element according to a certain aspect of the presentinvention;

FIG. 10 is a graph of a voltage-luminance characteristic of alight-emitting element according to a certain aspect of the presentinvention;

FIG. 11 is a graph of a luminance-current efficiency characteristic: ofa light-emitting element according to a certain aspect of the presentinvention; and

FIG. 12 is a graph of a light-emitting spectrum of a light-emittingelement according to a certain aspect of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiment Mode and Embodiments of the present invention are hereinafterdescribed with reference to the drawings. However, the invention is notlimited to the following description. It is to be understood by thoseskilled in the art that the modes and details of the invention can bechanged and modified without departing from the scope of the invention.Therefore, the invention is not limited to the description of thefollowing Embodiment Mode and Embodiments.

(Embodiment Mode)

FIG. 1 schematically shows an element structure of a light-emittingelement in the present invention. In the light-emitting element of theinvention, a first layer 102, a second layer 103, a third layer 104, anda fourth layer 105 are provided between a first electrode 101 and asecond electrode 106 in order from the first electrode 101 toward thesecond electrode 106.

In this embodiment mode, the second electrode 106 is formed with metal,and light emitted from the first layer 102 is extracted from the firstelectrode side. Light can be emitted by applying higher potential to thefirst electrode 101 than that of the second electrode 106.

The first electrode 101 is preferably formed with a light-transmittingmaterial, specifically indium tin oxide (ITO), indium tin oxidecontaining silicon oxide (ITSO), zinc oxide (ZnO), indium oxidecontaining zinc oxide by 2% to 20% (IZO), zinc oxide containing gallium(GZO), tin oxide (SnO₂), indium oxide (In₂O₃), or the like.

The first layer 102 is a layer containing a light-emitting material andis formed with a known material. The first layer 102 may be formed ineither a single-layer structure or a multilayer structure. For example,not only the light-emitting layer but also each of a functional layersuch as an electron-injecting layer, an electron-transporting layer, ahole-blocking layer, a hole-transporting layer, a hole-injecting layer,or the like may be freely combined and provided as the first layer 102.Moreover, a mixed layer or mixed junction in which each of these layersis mixed may be formed. The layer structure of the light-emitting layeris changeable. Such modification as providing an electrode for theelectron injection and combining this function in anelectron-transporting region instead of the particularelectron-injecting region is allowable within the scope of theinvention.

The second layer 103 contains a material having a donor level.Specifically, it is sufficient that the second layer 103 consists of orcontains an n-type semiconductor such as zinc oxide, tin oxide, titaniumoxide, zinc sulfide, zinc selenide, or zinc telluride. Alternatively,the second layer 103 may have a structure doped with anelectron-supplying material to an organic compound. The organic compoundhere is preferably an electron-transporting material such as2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviated toPBD), OXD-7, TAZ, p-EtTAZ, BPhen, or BCP. Besides, a metal complexhaving a quinoline skeleton or benzoquinoline skeleton in which thedrive voltage has been conventionally increased, such as Alq₃,tris(4-methyl-8-quinolinolato)aluminum (abbreviated to Almq₃), orbis(10-hydroxybenzo[h]-quinolinato)beryllium (abbreviated to BeBq₂), orbis(2-methyl-8-quinolinolato)-4-phenylphenolato-aluminum (abbreviated toBAlq) is given. On the other hand, the electron-supplying material is,for example, alkali metal such as Li or Cs, Mg, alkali-earth metal suchas Ca, Sr, or Ba, or rare-earth metal such as Er or Yb. In addition, forexample, in the case of using to Alq₃ as an organic compound, an organiccompound having electron-supplying properties such as tetrathiafulvaleneor tetra methyl thiafulvalene may also be doped as an electron-supplyingmaterial to Alq₃. Moreover, metal oxide such as molybdenum oxide,vanadium oxide, rhenium oxide, zinc oxide, tin oxide, or titanium oxidemay be mixed in a structure in which an electron-supplying material isdoped into an organic compound.

A layer containing a plurality of materials such as the layer includingthe material doped with an electron-supplying material to an organiccompound can be formed by depositing each material simultaneously. Thelayer containing a plurality of materials is desirably formed bycombining the same kinds of methods or the different kinds of methodssuch as a co-evaporation method with resistance heating evaporationmethods, a co-evaporation method with electron beam evaporation methods,a co-evaporation method with a resistance heating evaporation method andan electron beam evaporation method, deposition with a resistanceheating evaporation method and a sputtering method, or deposition withan electron beam evaporation method and a sputtering method. Inaddition, although a layer containing two kinds of materials are assumedin the above examples, the layer containing a plurality of materials canbe similarly formed in the case of containing three or more kinds ofmaterials.

The third layer 104 has light-transmitting properties and contains acarrier-generating material. Specifically, a transparent conductive filmsuch as indium tin oxide (ITO), indium tin oxide containing siliconoxide (ITSO), zinc oxide (ZnO), indium oxide containing zinc oxide by 2%to 20% (IZO), zinc oxide containing gallium (GZO), tin oxide (SnO₂),indium oxide (In₂O₃), metal oxide such as molybdenum oxide (MoO_(x)) andthe mixture thereof, or metal or alloy thinned enough to havelight-transmitting properties (for example, aluminum, silver, or thelike) can be used. When the metal thinned enough to havelight-transmitting properties is used as the transparent conductivefilm, the invention is applied in the case of a material different fromthe first electrode.

The fourth layer 105 contains a hole-transporting medium. Thehole-transporting medium is, for example, a hole-transporting materialcontaining an organic compound, a material doped with anelectron-receiving material to an organic compound, or ahole-transporting material containing an inorganic compound. The fourthlayer 105 can be formed with these hole-transporting mediums, and it ispreferable to use a material having an acceptor level for generatingholes, that is, a material doped with an electron-receiving material toan organic compound or a hole-transporting material containing aninorganic compound.

When the fourth layer contains a hole-transporting material containingan organic compound, the hole-transporting material to be used ispreferably a material having an aromatic-amine skeleton (that is, acompound having a bond of a benzene ring-nitrogen). For example, thefollowing material is widely used:N,N′-bis(3-metylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviated to TPD), a derivative thereof such as4,4′-bis[N-(1-naphtyl)-N-phenyl-amino]-biphenyl (abbreviated to α-NPD),or a starburst aromatic amine compound such as4,4′,4″-tris(N-carbazolyl)-triphenylamine (abbreviated to TCTA),4,4′,4″-tris(N,N-diphenyl-amino)-triphenylamine (abbreviated to TDATA),or 4,4′,4″-tris[(N-(3-methylphenyl)-N-phenyl-amino]triphenylamine(abbreviated to MTDATA).

Moreover, when the fourth layer 105 has a structure doped with amaterial having electron-receiving properties to an organic compound,the organic compound to be used is preferably a hole-transportingmaterial, and a material having an aromatic-amine skeleton is preferred.For example, in addition to TPD, α-NPD (derivative of TPD) or astarburst aromatic amine compound such as TDATA or MTDATA is given. Onthe other hand, for example, metal oxide having electron-receivingproperties to α-NPD, such as molybdenum oxide, vanadium oxide, orrhenium oxide is given as the electron-receiving material. Moreover,another organic compound having electron-receiving properties to α-NPDsuch as tetracyanoquinodimethane (abbreviated to TCNQ) or2,3-dicyanonaphthoquinone (abbreviated to DCNNQ) may also be used.

A layer containing a plurality of materials such as the structureincluding the material doped with an electron-receiving material to anorganic compound can be formed by depositing each materialsimultaneously. The layer containing a plurality of materials isdesirably formed by combining the same kinds of methods or the differentkinds of methods such as a co-evaporation method with resistance heatingevaporation methods, a co-evaporation method with electron beamevaporation methods, a co-evaporation method with a resistance heatingevaporation method and an electron beam evaporation method, depositionwith a resistance heating evaporation method and a sputtering method, ordeposition with an electron beam evaporation method and a sputteringmethod. In addition, although a layer containing two kinds of materialsare assumed in the above examples, the layer containing a plurality ofmaterials can be similarly formed in the case of containing three ormore kinds of materials:

When the fourth layer 105 contains a hole-transporting materialcontaining an inorganic compound, it is sufficient that the fourth layer105 consists of or contains a p-type semiconductor such as vanadiumoxide, chromium oxide, molybdenum oxide, cobalt oxide, or nickel oxide.Note that the fourth layer 105 is made to be formed with a materialdifferent from that used for the third layer 104.

The second electrode 106 is preferably formed with metal having highreflectivity. For example, aluminum (Al), silver (Ag), or an alloycontaining Al or Ag such as an AlLi alloy or an MgAg alloy can be used.In addition, the second electrode 106 may be formed in a multilayerstructure of reflective metal and another electrode material. Theelectron injectability can be increased by forming a multilayer ofreflective metal and a thin film (for example of approximately 5 nmthick) of alkali metal or alkali-earth metal.

According to the structure shown in this embodiment mode, the fourthlayer 105 is provided between the second electrode 106 and the thirdlayer 104, and the second electrode 106 including reflective metal andthe third layer 104 including the transparent conductive film are not indirect contact. Therefore, electric erosion due to the difference in theself-potential can be prevented. That is to say, the reaction of themetal and the transparent conductive film can be prevented.

Not only the third layer 104 including the transparent conductive filmbut also the fourth layer 105 can have any thickness. Therefore, thedegree of freedom to optimize the optical length L to the reflectivemetal from the first layer 102 containing a light-emitting materialincreases further. For this reason, the optical length can be optimizedmore easily so as to increase the external quantum efficiency or toincrease the color purity of the emission light.

Since the first layer 102, the second layer 103, the third layer 104,the fourth layer 105, and the second electrode 106 are stacked, holesand electrons can be generated from the third layer. Since the secondlayer. 103 contains a material having a donor level for generatingelectrons, the electrons generated from the third layer 104 has thesmall barrier for electrons to move from the third layer 104 to thesecond layer 103. Therefore; the electrons are easily moved to thesecond layer 103 and recombined with the holes injected from the firstelectrode in the first layer 102, thereby emitting light. On the otherhand, the holes generated from the third layer 104 including thetransparent conductive film has the small barrier for holes to move fromthe third layer 104 to the fourth layer 105 containing thehole-transporting medium; therefore, the holes are easily moved to thefourth layer and transported to the second electrode 106.

That is to say, in the structure according to the invention, thesubstantial moving distance of the electrons can be shortened, whichenables the drive voltage to decrease. Therefore, when the opticallength is optimized to increase the external quantum efficiency or thecolor purity and the distance to the reflective metal from the layercontaining the light-emitting material is set to a certain distance, thesubstantial moving distance of the electrons can be shortened byemploying the present invention, which enables the drive voltage todecrease.

In addition, even in the case of extending the distance to the metalfrom the layer containing the light-emitting material and increasing thefilm thickness in order to optimize the optical length, the increase inthe drive voltage can be suppressed.

The contact resistance between the second layer 103 and the fourth layer105 can be reduced by stacking the second layer 103 and the fourth layer105 with the third layer 104 sandwiched therebetween. This makes itpossible to further decrease the drive voltage. Since the third layer104 is sandwiched therebetween, each of the materials for the secondlayer 103 and the fourth layer 105 can be selected from a wider range.

The contact resistance between the second layer 103 and the third layer104 and the contact resistance between the third layer 104 and thefourth layer 105 are preferably low.

[Embodiment 1]

This embodiment explains a structure of a light-emitting elementaccording to the present invention with reference to FIG. 3.

First, a first electrode 301 of a light-emitting element is formed overa substrate 300. Specifically, the first electrode 301 is formed withITO, a transparent conductive film, in 110 nm thick by a sputteringmethod. The first electrode 301 has a length of 2 mm on a side.

Next, a first layer 302 containing a light-emitting material is formedover the first electrode 301. The first layer 302 containing alight-emitting material in this embodiment has a multilayer structureincluding three layers 311, 312, and 313.

The substrate with the first electrode 301 formed thereover is fixedonto a substrate holder in a vacuum evaporation apparatus in such a waythat the surface of the substrate with the first electrode 301 formedfaces downward, and copper phthalocyanine (hereinafter referred to asCu-Pc) is introduced into an evaporation source equipped inside thevacuum evaporation apparatus. Then, a hole-injecting layer 311containing a hole-injecting material is formed in 20 nm thick by anevaporation method using a resistance heating method. A knownhole-injecting material can be used as the material for thehole-injecting layer 311.

Next, a hole-transporting layer 312 is formed with a material superiorin hole transportability. A known hole-transporting material can be usedas the material for the hole-transporting layer 312. In this embodiment,α-NPD is formed in 40 nm thick by the same method.

Then, a light-emitting layer 313 is formed. A known light-emittingmaterial can be used as the material for the light-emitting layer 313.In this embodiment, Alq₃ is formed in 40 am thick by the same method.

In this manner, the three layers 311, 312, and 313 are stacked. Next, asecond layer 303 is formed. In this embodiment, the second layer 303 isformed in 30 nm thick by a co-evaporation method by resistance heatingin such a way that Alq₃ is used as an electron-transporting material(host material) and Mg is used as an electron-supplying material (guestmaterial) to Alq₃. The proportion of the guest material is set 1 mass %.

Subsequently, a third layer 304 is formed. In this embodiment, ITO isused to form a transparent conductive layer in 140 nm thick.

Next, a fourth layer 305 is formed. In this embodiment, the fourth layer305 is formed in 150 nm thick by a co-evaporation method by resistanceheating in such a way that α-NPD is used as a hole-transporting material(host material) and molybdenum oxide is used as an electron-receivingmaterial (guest material) to α-NPD. The proportion of the guest materialis set 25 mass %.

Then, a second electrode 306 is formed by a sputtering method or anevaporation method. In this embodiment, the second electrode 306 isobtained by forming aluminum in 150 nm thick over the fourth layer 305by an evaporation method.

Through the above steps, a light-emitting element of the invention isformed. In the structure shown in this embodiment, light can be emittedby applying higher potential to the first electrode 101 than that of thesecond electrode 106 and light generated by the recombination of thecarriers in the first layer containing a light-emitting material isemitted from the first electrode 301 to the outside.

In the structure shown in this embodiment, the fourth layer is providedbetween ITO serving as the third layer and aluminum serving as thesecond electrode; therefore, the ITO and the aluminum are not in directcontact. This can prevent the electric erosion due to the difference inthe self-potential between ITO and aluminum.

In addition, since the thicknesses of the third layer and the fourthlayer can be set freely, the optical length from the first layer to thesecond electrode formed with the reflective metal can be optimized moreeasily.

Moreover, since the carriers can be generated from the third layer, themoving distance of the electrons is shorter than that in the elementhaving a conventional structure. Therefore, the drive voltage can bedecreased.

[Embodiment 2]

This embodiment explains a structure of a light-emitting elementaccording to the present invention with reference to FIG. 4.

Since a substrate 400, a first electrode 401, a first layer 402, asecond layer 403, a third layer 404, a fourth layer 405, and a secondelectrode 406 can be formed with the same material and in the same wayas those in Embodiment Mode 1; the explanation is omitted. Even in thisstructure, light is emitted by applying higher potential to the firstelectrode 401 than that of the second electrode 406.

In addition, FIG. 4 has a structure including the second electrode 406formed over the substrate 400, the fourth layer 405 formed over thesecond electrode 406, the third layer 404 formed over the fourth layer405, the second layer 403 formed over the third layer 404, the firstlayer 402 containing a light-emitting material formed over the secondlayer 403, and the first electrode 401 formed over the first layer 402.

In the structure shown in this embodiment, light generated by therecombination of the carriers in the first layer containing alight-emitting material is emitted from the first electrode 401 to theoutside.

Even in this structure shown in this embodiment, the same advantage asthat obtained by the structure shown in Embodiment 1 can be obtained.Specifically, since the fourth layer is provided between the third layerand the second electrode, the electric erosion due to the difference inthe self-potential can be prevented. In addition, since the filmthicknesses of the third layer and the fourth layer can be set freely,the optical length from the first layer to the second electrode formedwith reflective metal can be optimized more easily. Moreover, the movingdistance of the electrons is shorter than that in the element having aconventional structure because the carrier can be generated from thethird layer; therefore, the drive voltage can be decreased.

[Embodiment 3]

This embodiment explains a light-emitting device having a light-emittingelement according to the present invention in its pixel portion withreference to FIGS. 5A and 5B. FIG. 5A is a top view showing thelight-emitting device, and FIG. 5B is a cross-sectional view taken alongA-A′ in FIG. 5A. Reference numeral 501 shown with a dotted line denotesa driver circuit portion (source driver circuit); 502, a pixel portion;503, a driver circuit portion (gate driver circuit); 504, a sealsubstrate; 505, a sealant, and 507, a space surrounded by the sealant505.

A lead wiring 508 denotes a wiring for transmitting signals to beinputted into the source driver circuit 501 and the gate driver circuit503, which receives signals such as a video signal, a clock signal, astart signal, and a reset signal from an FPC (flexible-printed circuit)509 serving as an external input terminal. Although only the FPC isshown here, a print wiring board (PWB) may be attached to this FPC andthe light-emitting device in this specification may include not only thelight-emitting device itself but also the light-emitting device with theFPC or the PWB attached thereto.

Next, the cross-sectional structure is explained with reference to FIG.5B. The driver circuit portion and the pixel portion are formed over asubstrate 510. In this embodiment, the source driver circuit 501, whichis the driver circuit portion, and the pixel portion 502 are shown.

In the source driver circuit 501, a CMOS circuit in which an n-channelTFT and a p-channel TFT 524 are combined is formed. In addition, a TFTfor forming the driver circuit may be formed with a known CMOS circuit,PMOS circuit, or NMOS circuit. Although this embodiment shows a driverintegrated type in which a driver circuit is formed over the samesubstrate, but not exclusively, the driver circuit can be formed outsidethe substrate.

The pixel portion 502 is formed with a plurality of pixels including aswitching TFT 511, a current control TFT 512, and a first electrode 513connected electrically with the drain of the current control TFT. Aninsulator 514 is formed so as to cover the end portion of the firstelectrode 513. Here, a positive photosensitive acrylic resin film isused as the insulator 514.

In order to improve the coverage, the insulator 514 is formed so as tohave curvature at its upper end or lower end. For example, in the caseof using positive photosensitive acrylic for the insulator 514, only theupper end portion of the insulator 514 preferably has a radius ofcurvature of 0.2 μm to 3 μm. The insulator 514 may be formed with eithera negative type, which becomes insoluble to the etchant by theirradiation of light, or a positive type, which becomes soluble to theetchant by the irradiation of light. Not only the organic compound butalso an inorganic compound such as silicon oxide or silicon oxynitridecan be used. In addition, a material composed of a skeleton structureformed by the bond of silicon and oxygen having as a substituent anorganic group at least containing hydrogen (such as an alkyl group oraryl group), a fluoro group, or an organic group at least containinghydrogen and a fluoro group, a so-called siloxane can also be used.

An electroluminescence layer 516 including first to fourth layers, and asecond electrode 517 are each formed over the first electrode 513. Thefirst electrode (anode) 513 is preferably formed with alight-transmitting material such as indium tin oxide (ITO), indium tinoxide containing silicon oxide (ITSO), zinc oxide (ZnO), indium oxidecontaining zinc oxide by 2% to 20% (IZO), zinc oxide containing gallium(GZO), tin oxide (SnO₂), or indium oxide (In₂O₃).

The first to fourth layers of the electroluminescence layer 516 are eachformed by resistance heating, an evaporation method by an electron beamboth using an evaporation mask, or an ink-jet method. The first tofourth layers of the electroluminescence layer 516 include a first layercontaining a light-emitting material, a second layer, a third layerincluding a transparent conductive film, and a fourth layer, in whichthe first layer, the second layer, the third layer, and the fourth layerare stacked sequentially from the first electrode toward the secondelectrode, and the fourth layer is formed so as to be in contact withthe second electrode. As the material for the layer containing thelight-emitting material, an organic compound is generally used in asingle-layer, multilayer, or a mixed-layer structure. However, in theinvention, an inorganic compound may also be used as a part of a filmcontaining the organic compound. In this case, deposition by asputtering method may be employed.

A layer containing a plurality of materials can be formed by depositingeach material simultaneously. The layer containing a plurality ofmaterials is desirably formed by combining the same kinds of methods orthe different kinds of methods such as a co-evaporation method withresistance heating evaporation methods, a co-evaporation method withelectron beam evaporation methods, a co-evaporation method with aresistance heating evaporation method and an electron beam evaporationmethod, deposition with a resistance heating evaporation method and asputtering method, or deposition with an electron beam evaporationmethod and a sputtering method. In addition, although a layer containingtwo kinds of materials are assumed in the above examples, the layercontaining a plurality of materials can be similarly formed in the caseof containing three or more kinds of materials.

Metal having high reflectivity is preferable as the material for thesecond electrode 517 (cathode) formed over the electroluminescence layer516. For example, aluminum (Al); silver (Ag), or alloy containing Al orAg-such as an AlLi alloy or an MgAg alloy can be used.

Further, a light-emitting element 518 is provided within the space 507surrounded with the element formed substrate 510, the sealing substrate504, and the sealant 505 by attaching the sealing substrate 504 to theelement formed substrate 510 with the sealant 505. The space 507 isfilled with a filling material, for example, inert gas such as nitrogenor argon, or the sealant 505.

An epoxy-based resin and the like are preferably used for the sealant505. It is desirable that these materials do not transmit oxygen ormoisture as much as possible. As the material for the sealing substrate504, a plastic substrate made of FRP (Fiberglass-Reinforced Plastics),PVF (polyvinyl fluoride), Mylar, polyester, acrylic, or the like as wellas a glass substrate or a quartz substrate can be used.

As thus described, a light-emitting device having a light-emittingelement according to the invention can be obtained.

This embodiment can be arbitrarily combined with other embodiment modeand embodiments.

[Embodiment 4]

This embodiment explains a structure of the above layer containing alight-emitting material in detail.

The layer containing a light-emitting material is formed from acharge-injecting-transporting material and a light-emitting materialcontaining an organic compound or an inorganic compound. The layercontaining a light-emitting material includes one or a plurality oflayers of a low molecular weight organic compound, a middle molecularweight organic compound, and a high molecular weight organic compound.Alternatively, an inorganic compound having electron-injectingtransportability or hole-injecting transportability may also becombined.

The following metal complex or the like having a quinoline skeleton or abenzoquinoline skeleton can be given particularly as an example of thematerial having high electron transportability among thecharge-injecting-transporting material: tris(8-quinolinolato)aluminum(abbreviated to Alq₃), tris(4-methyl-8-quinolinolato)aluminum(abbreviated to Almq₃), bis(10-hydroxybenzo[h]-quinolinato)beryllium(abbreviated to BeBq₂),bis(2-methyl-8-quinolinolato)-4-phenylphenolato-aluminum (abbreviated toBAlq) or the like. In addition, the following material having anaromatic-amine skeleton (that is, a material having a benzenering-nitrogen bond) can be given as an example of the material havinghigh hole transportability:4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl (abbreviated to α-NPD),4,4′-bis[N-(3-methylphenyl)-N-phenyl-amino]biphenyl (abbreviated toTPD), 4,4′,4″-tris(N,N-diphenyl-amino)-triphenylamine (abbreviated toTDATA), 4,4′,4″-tris[N-(3-methylphenyl)-N-phenyl-amino]-triphenylamine(abbreviated to MTDATA), or the like.

Moreover, a compound of alkaline metal or alkaline earth metal such aslithium fluoride (LiF), cesium fluoride (CsF), or calcium fluoride(CaF₂) can be given particularly as an example of the material havinghigh electron injectability among the charge-injecting-transportingmaterial. Additionally, a mixture of a substance having high electrontransportability such as Alq₃ and an alkaline earth metal such asmagnesium (Mg) may also be applied.

The following metal oxide can be given as an example of the materialhaving high hole injectability among the charge-injecting-transportingmaterial: molybdenum oxide (MoO_(x)), vanadium oxide (VO_(x)), rutheniumoxide (RuO_(x)), tungsten oxide (WO_(x)), manganese oxide (MnO_(x)), orthe like. Additionally, a phthalocyanine-based compound such asphthalocyanine (abbreviated to H₂Pc) or copper phthalocyanine (CuPc) canbe given as an example.

A light-emitting layer may have a structure performing a color displayby forming a light-emitting layer having different emission wavelengthranges in each pixel. Typically, a light-emitting layer corresponding toeach color of R (red), G (green) and B (blue) is formed. In this case,color purity can also be improved and a mirror surface of a pixelportion can be prevented by having a structure provided with a filter(colored layer) in which light in the emission wavelength range istransmitted on the side where light from a pixel is emitted. It ispossible to skip providing a circular polarizing plate or the like whichis conventionally necessary by providing the filter (colored layer), andthis can prevent loss of light emitted from the light-emitting layer.Further, there can be less variation of color tone generated in the caseof obliquely seeing the pixel portion (a display screen).

There are various light-emitting materials which form the light-emittinglayer. The following can be used as a low molecular weight organiclight-emitting material:4-dicyanomethylene-2-methyl-6-[2-(1,1,7,7-tetramethyljulolidine-9-yl)ethenyl]-4H-pyran(abbreviated to DCJT),4-dicyanomethylene-2-t-butyl-6-[2-(1,1,7,7-tetramethyljulolidine-9-yl)ethenyl]-4H-pyran,periflanthen,2,5-dicyano-1,4-bis[2-(10-methoxy-1,1,7,7-tetramethyljulolidine-9-yl)ethenyl]benzene,N,N′-dimethylquinacridon (abbreviated to DMQd), coumarin6, coumarin545T,tris(8-quinolinolato)aluminum (abbreviated to Alq₃), 9,9′-bianthryl,9,10-diphenylanthracene (abbreviated as DPA),9,10-bis(2-naphthyl)anthracene (abbreviated to DNA), or the like.Alternatively, another material may also be used.

On the other hand, a high molecular weight organic light-emittingmaterial has higher physical strength and higher durability of anelement compared with the low molecular weight organic light emittingmaterial. In addition, since the film-forming by coating is possible,manufacturing the element is comparatively easy. The structure of alight-emitting element using the high molecular weight organiclight-emitting material is basically the same as in the case of usingthe low molecular weight organic light-emitting material, in which acathode, an organic light-emitting layer, and an anode are stacked.However, in forming a light-emitting layer using the high molecularweight organic light-emitting material, it is difficult to form amultilayer structure as in the case of using the low molecular weightorganic light-emitting material; thus, a two-layer structure is formedin many cases. Specifically, a cathode, a light-emitting layer, ahole-transporting layer, and an anode are stacked in the multilayerstructure.

Since luminescence color depends on materials which form alight-emitting layer, a light-emitting element that shows a desiredluminescence by selecting these materials can be formed.Polyparaphenylene vinylene based, polyparaphenylene based, polythiophenebased and polyfluorene based light-emitting materials are given as anexample of a high molecular weight electroluminescence material whichcan be used to form a light-emitting layer.

The following can be given as an example of the polyparaphenylenevinylene based light-emitting material: a derivative ofpoly(paraphenylenevinylene) [PPV],poly(2,5-dialkoxy-1,4-phenylenevinylene) [RO-PPV],poly(2-(2′-ethyl-hexoxy)-5-methoxy-1,4-phenylenevinylene) [MEH-PPV],poly(2-(dialkoxyphenyl)-1,4-phenylenevinylene) [ROPh-PPV], or the like.The following can be given as an example of the polyparaphenylene basedlight-emitting material: a derivative of polyparaphenylene [PPP],poly(2,5-dialkoxy-1,4-phenylene) [RO-PPP],poly(2,5-dihexoxy-1,4-phenylene), or the like. The following can begiven as an example of the polythiophene based light-emitting material:a derivative of polythiophene [PT], poly(3-alkylthiophene) [PAT],poly(3-hexylthiophene) [PHT], poly(3-cyclohexylthiophene) [PCHT],poly(3-cyclohexyl-4-methylthiophene) [PCHMT],poly(3,4-dicyclohexylthiophene) [PDCHT],poly[3-(4-octylphenyl)-thiophene] [POPT],poly[3-(4-octylphenyl)-2,2bithiophene][PTOPT], or the like. Thefollowing can be given as an example of the polyfluorene basedlight-emitting material: a derivative of polyfluorene [PF],poly(9,9-dialkylfluorene) [PDAF], poly(9,9-dioctylfluorene) [PDOF], orthe like.

Note that the hole injectability from an anode can be enhanced when ahigh molecular weight organic light-emitting material having holetransportability is sandwiched between an anode and a high molecularorganic light-emitting material having light-emitting properties.Generally, a high molecular weight organic light-emitting material iscoated with a solution in which an acceptor material is dissolved inwater by a spin-coating method or the like. In addition, since theacceptor material is insoluble in an organic solvent, the organiclight-emitting material having light-emitting properties mentioned abovecan be stacked therewith. A mixture of PEDOT and camphor sulfonic acid(CSA) as an acceptor material, a mixture of polyaniline [PANI] andpolystyrenesulphonic [PSS] as an acceptor material or the like can begiven as an example of the high molecular weight organic light-emittingmaterial having hole transportability.

In addition, a light-emitting layer can have a structure emittingmonochromatic or white light emission. The case of using a whitelight-emitting material enables a color display by constituting astructure provided with a filter (a colored layer) transmitting lighthaving a particular wavelength on the side where light from a pixel isemitted.

In order to form a light-emitting layer having white light emission, forexample, Alq₃, Alq₃ partially doped with Nile Red, which is a red lightemitting pigment, Alq₃, p-EtTAZ, and TPD (aromatic diamine) are stackedsequentially by an evaporation method, thereby being able to obtainwhite light emission. In addition, in the case of forming an EL by acoating method using spin coating, the EL layer is preferably baked byvacuum heating after the coating. For example, an entire surface may becoated with a poly (ethylenedioxythiophene)/poly (styrenesulfonic acid)solution (PEDOT/PSS) and baked in order to form a film that serves as ahole-injecting layer. Thereafter, the entire surface may be coated witha polyvinyl carbazole (PVK) solution doped with luminescent centerpigment (such as 1,1,4,4-tetraphenyl-1,3-butadiene (TPB),4-dicyanomethylene-2-methyl-6-(p-dimethylamino-styryl)-4H-pyran (DCM1),Nile red, or coumarin 6) and baked in order to form a film that servesas a light emitting layer.

A light-emitting layer can be formed in a single layer, and a1,3,4-oxadiazole derivative (PBD) having electron transportability maybe dispersed in polyvinyl carbazole (PVK) having hole transportability.In addition, white light emission can be obtained by dispersing PBD by30 wt % as an electron-transporting agent and dispersing appropriateddoses of four kinds of dyes (TPB, coumarin6, DCM1 and Nile Red). Inaddition to the light-emitting element with which white light emissionis obtained, as shown here, a light-emitting element that can obtain redlight emission, green light emission, or blue light emission can bemanufactured by selecting the materials of the light-emitting layerappropriately.

Further, a triplet excited light-emitting material including a metalcomplex or the like may be used for the light-emitting layer in additionto a singlet excited light-emitting material. For example, among a pixelhaving red light-emitting properties, a pixel having greenlight-emitting properties and a pixel having blue light-emittingproperties, the pixel having red light-emitting properties withcomparatively short half reduced luminescence time is formed from atriplet excited light-emitting material, and other pixels are formedfrom a singlet excited light-emitting material. Since the tripletexcited light-emitting material has preferable luminous efficiency,there is a feature that less power consumption is required to obtain thesame luminance. In other words, in the case of applying the tripletexcited light-emitting material to the red pixel, a few amount ofcurrent flown to a light-emitting element is required; therefore, thereliability can be enhanced. The pixel having red light-emittingproperties and the pixel having green light-emitting properties may beformed from a triplet excited light-emitting material, and the pixelhaving blue light-emitting properties may be formed from a singletexcited light-emitting material to reduce the power consumption. Furtherlow power consumption can be realized by also forming the greenlight-emitting element, which has high human spectral luminous efficacy,from a triplet excited light-emitting material.

A metal complex used as a dopant, a metal complex in which platinum,which is a third transition series element, serves as a center metal, ametal complex in which iridium serves as a center metal, or the like isknown as an example of a triplet excited light-emitting material. Thetriplet excited light-emitting material is not limited to thesecompounds and it is also possible to use a compound having the abovestructure and having an element belonging to Groups 8 to 10 of aperiodic table for a center metal.

The substances mentioned above that forms the light-emitting layer arejust an example, and a light-emitting element can be formed by stackingappropriately each functional layer such as ahole-injecting-transporting layer, a hole-transporting layer, anelectron-injecting-transporting layer, an electron-transporting layer, alight-emitting layer, an electron-blocking layer, or a hole-blockinglayer. Moreover, a mixed layer or mixed junction in which each of theselayers is mixed may be formed. The layer structure of the light-emittinglayer is changeable. Such modification as providing an electrode for theelectron injection and providing a light-emitting material by beingdispersed instead of the particular electron-injecting region and thelight-emitting region is allowable within the scope of the invention.

The light-emitting element formed by using the material mentioned aboveemits light by being biased in a forward direction. A pixel of a displaydevice formed by using the light-emitting element can be driven by asimple matrix mode or an active matrix mode. In any event, each pixel isemitted by applying a forward bias thereto in specific timing; however,the pixel is in a non-luminescent state for a certain period.Reliability of a light-emitting element can be enhanced by applying biasin the opposite direction (a reverse bias) during this non-luminescentperiod. In a light-emitting element, there is a deterioration mode inwhich emission intensity is decreased under a certain driving conditionor a deterioration mode in which luminance is apparently decreased dueto the expansion of a non-luminescent region in the pixel. However, theprogression of deterioration can be delayed by alternating currentdriving. Accordingly, reliability of a light emitting device can beenhanced.

This embodiment can be arbitrarily combined with other embodiment modeand embodiments.

[Embodiment 5]

This embodiment explains a module installed with such a light-emittingdevice shown in Embodiment 3.

In a module 999 of an information terminal shown in FIG. 7A, acontroller 901, a central processing unit (CPU) 902, a memory 911, apower supply circuit 903, an audio processing circuit 929, and atransmitter/receiver circuit 904 as well as other elements such as aresistor, a buffer, and a capacitor element are mounted on aprinted-wiring board 946. In addition, a display panel 900 including alight-emitting device is connected to the printed-wiring board 946through a flexible-printed circuit (FPC) 908.

The display panel 900 includes a pixel portion 905 where alight-emitting element is disposed in each pixel, a first scanning linedriver circuit 906 a and a second scanning line-driver circuit 906 beach for selecting a pixel included in the pixel portion 905, and asignal line driver circuit 907 for supplying a video signal to theselected pixel.

Various control signals are inputted and outputted through an interface(I/F) portion 909 provided for the printed-wiring board 946. Inaddition, the printed-wiring board 946 is provided with an antenna port910 for transmitting/receiving signals to/from the antenna.

Although the printed-wiring board 946 is connected to the display panel900 through the FPC 908 in this embodiment, the invention is notnecessarily limited to this structure. The controller 901, the audioprocessing circuit 929, the memory 911, the CPU 902, or the power supplycircuit 903 may be mounted directly on the display panel 900 by using aCOG (Chip on Glass) method. In addition, the printed-wiring board 946 isprovided with various elements such as a capacitor element or a buffer,which prevents noise from causing in the power supply voltage orsignals, or the rise of the signal from becoming slow.

FIG. 7B shows a block diagram of the module 999 shown in FIG. 7A. Thismodule 999 includes a VRAM 932, a DRAM 925, a flash memory 926, and thelike as the memory 911. Data of an image to be displayed in the panelare stored in the VRAM 932, image data or audio data are stored in theDRAM 925, and various programs are stored in the flash memory 926.

In the power supply circuit 903, power supply voltage for the displaypanel 900, the controller 901, the CPU 902, the audio processing circuit929, the memory 911, and the transmitter/receiver circuit 931 isgenerated. In some cases, the power supply circuit 903 is provided witha current source depending on the panel specification.

The CPU 902 includes a control signal generating circuit 920, a decoder921, a register 922, an operation circuit 923, a RAM 924, an interface935 for CPU, and the like. The various signals inputted into the CPU 902through the interface 935 are once stored in the register 922, and then,are inputted into the operation circuit 923, the decoder 921, and thelike. In the operation circuit 923, an operation is performed based onthe inputted signal, and a place to which various instructions aretransmitted is designated. On the other hand, the signal inputted intothe decoder 921 is decoded and is inputted into the control signalgenerating circuit 920. A signal including various instructions isgenerated in the control signal generating circuit 920 base on theinputted signal and is transmitted to the place designated by theoperation circuit 923, specifically, to the memory 911, thetransmitter/receiver circuit 931, the audio processing circuit 929, thecontroller, or the like.

The memory 911, the transmitter/receiver circuit 931, the audioprocessing circuit 929, and the controller 901 each operate according toeach of the received instructions. Each operation is briefly explainedhereinafter.

A signal inputted from an input means 933 is transmitted to the CPU 902which is mounted on the printed-wiring board 946 through the interface909. In the control signal generating circuit 920, the image data storedin the VRAM 932 is converted to a predetermined format in accordancewith the signal transmitted from the input means 933 such as a pointingdevice or a keyboard and is sent to the controller 901.

A signal including the image data transmitted from the CPU 902 isdata-processed in accordance with the panel specification in thecontroller 901 and is supplied to the display panel 900. In addition, aHsync signal, a Vsync signal, a clock signal CLK, a volts alternatingcurrent (AC Cont), and a switching-over signal L/R are generated in thecontroller 901 in accordance with a power supply voltage inputted fromthe power supply circuit 903 or various signals inputted from the CPU902 and are supplied to the display panel 900.

In the transmitter/receiver circuit 904, a signal that is transmittedand received as an electric wave in an antenna 934 is processed, andspecifically, a high frequency circuit such as an isolator, a band passfilter, a VCO (Voltage Controlled Oscillator), an LPF (Low Pass Filter),a coupler, and a balun is included. A signal including audio informationamong the signals transmitted and received in the transmitter/receivercircuit 904 is transmitted to the audio processing circuit 929 by aninstruction of the CPU 902.

A signal including audio information transmitted by the instruction ofthe CPU 902 is demodulated into an audio signal in the audio processingcircuit 929 and is transmitted to a speaker 928. An audio signaltransmitted from a microphone 927 is modulated in the audio processingcircuit 929 and is transmitted to the transmitter/receiver circuit 904by the instruction of the CPU 902.

The controller 901, the CPU 902, the power supply circuit 903, the audioprocessing circuit 929, and the memory 911 can be mounted as a packageof this embodiment. This embodiment can be applied to anything but ahigh frequency circuit such as an isolator, a band pass filter, a VCO(Voltage Controlled Oscillator), an LPF (Low Pass Filter), a coupler, ora balun.

In the display panel 900, a transparent conductive film is not in directcontact with a reflective electrode with optimization of the opticallength in its light-emitting element. Therefore, the electric erosion ofthe transparent conductive film and the reflective electrode can beprevented. Accordingly, in the module 999 equipped with this displaypanel 900, the deterioration due to the electric erosion can bedecreased and display quality is improved as well. Therefore, it ispossible to provide a module having preferable display quality, highreliability, and long life time.

This embodiment can be arbitrarily combined with other embodiment modeand embodiments.

[Embodiment 6]

FIG. 8 shows one mode of an electronic device including the module 999as shown in Embodiment 5. A display panel 900 is capable of beingintegrated with the module 999 easily by being incorporated into ahousing 1001 in a free desorption manner. The shape or dimension of thehousing 1001 can be changed arbitrarily according to the electronicdevice in which the housing 1001 is incorporated.

The housing 1001 that fixed the display panel 900 is inserted in aprinted-wiring board 946 and is built up as a module. A controller, aCPU, a memory, and a power supply circuit as well as other elements suchas a resistor, a buffer, and a capacitor element are mounted on theprinted-wiring board 946. Further, an audio processing circuit, atransmitter/receiver circuit, or the like may be mounted depending onthe purpose. The display panel 900 is connected to the printed-wiringboard 946 through an FPC 908.

Such module 999, an input means 998, and a battery 997 are held incasings 996. A pixel portion of the display panel 900 is disposed sothat it is visible from an opening window formed in the casing 996.

In the display panel 900, a transparent conductive film is not in directcontact with a reflective electrode with optimization of the opticallength in its light-emitting element. Therefore, the electric erosion ofthe transparent conductive film and the reflective electrode can beprevented. Accordingly, in the module 999 equipped with this displaypanel 900, the deterioration due to the electric erosion can bedecreased and display quality is improved as well. Therefore, it ispossible to provide a cellular phone having preferable display quality,high reliability, and long life time.

[Embodiment 7]

This embodiment explains a mode of an electronic device on which themodule as shown in Embodiment 5 is mounted, which is different from thatshown in Embodiment 6.

Electronic devices manufactured using light-emitting devices havinglight-emitting elements according to the present invention include acamera such as a video camera or a digital camera, a goggle type display(head mounted display), a navigation system, an audio reproducing device(such as a car audio or an audio component), a personal computer, a gamemachine, a portable information terminal (a mobile computer, a cellularphone, a portable game machine, an electronic book, or the like), animage reproducing device provided with a recording medium (specificallya device capable of playing a recording medium such as a DigitalVersatile Disc (DVD) and that has a display device capable of displayingthe image), or the like. These electronic devices are specifically shownin FIGS. 6A to 6E.

FIG. 6A is a television receiver, which includes a casing 9101, asupporting stand 9102, a display portion 9103, speaker portions 9104, avideo input terminal 9105, and the like. The television receiver ismanufactured by using the light-emitting device having a light-emittingelement of the present invention for its display portion 9103. Areflective metal and a transparent conductive film are not in directcontact with optimization of the optical length in the light-emittingelement of the display portion 9103. Therefore, the electric erosion dueto the difference in the self-potentials can be prevented; thus, thereliability of the television receiver is improved. Note that thetelevision receiver includes all the information display devices for acomputer, TV broadcast reception, advertisement display, and the like.

FIG. 6B is a personal computer, which includes a main body 9201, acasing 9202, a display portion 9203, a keyboard 9204, an externalconnection port 9205, a pointing mouse 9206, and the like. The personalcomputer is manufactured by using the light-emitting device having alight-emitting element of the invention for its display portion 9203. Areflective metal and a transparent conductive film are not in directcontact with optimization of the optical length in the light-emittingelement of the display portion 9203. Therefore, the electric erosion dueto the difference in the self-potentials can be prevented; thus, thereliability of the personal computer is improved.

FIG. 6C is a goggle-type display, which includes a main body 9301,display portions 9302, arm portions 9303, and the like. The goggle-typedisplay is manufactured by using the light-emitting device having alight-emitting element of the invention for its display portion 9302. Areflective metal and a transparent conductive film are not in directcontact with optimization of the optical length in the light-emittingelement of the display portion 9302 Therefore, the electric erosion dueto the difference in the self-potentials can be prevented; thus, thereliability of the goggle-type display is improved.

FIG. 6D is a cellular phone, which includes a main body 9401, a casing9402, a display portion 9403, an audio input portion 9404, an audiooutput portion 9405, operation keys 9406, an external connection port9407, an antenna 9408, and the like. The cellular phone is manufacturedby using the light-emitting device having a light-emitting element ofthe invention for its display portion 9403. A reflective metal and atransparent conductive film are not in direct contact with optimizationof the optical length in the light-emitting element of the displayportion 9403. Therefore, the electric erosion due to the difference inthe self-potentials can be prevented; thus, the reliability of thecellular phone is improved. Note that the power consumption of thecellular phone can be suppressed by displaying White letters on a blackbackground of the display portion 9403.

FIG. 6E is a video camera, which includes a main body 9501, a displayportion 9502, a casing 9503, an external connection port 9504, a remotecontrol receiving portion 9505, an image receiving portion 9506, abattery 9507, an audio input portion 9508, operation keys 9509, aneyepiece portion 9510, and the like. The video camera is manufactured byusing the light-emitting device having a light-emitting element of theinvention for its display portion 9502. A reflective metal and atransparent conductive film are not in direct contact with optimizationof the optical length in the light-emitting element of the displayportion 9502. Therefore, the electric erosion due to the difference inthe self-potentials can be prevented; thus, the reliability of the videocamera is improved.

As mentioned above, the light-emitting device having a light-emittingelement according to the invention can be applied in an extremely widerange, and this light-emitting device can be applied to electronicdevices of every field. By using the light-emitting element of theinvention, the optical length to the reflective metal from the layercontaining a light-emitting material can be optimized without increasingthe drive voltage.

This embodiment can be arbitrarily combined with other embodiment modeand embodiments.

[Embodiment 8]

FIGS. 9 to 12 each show characteristics of a light-emitting element ofthe present invention in a graph. FIG. 9 shows a currentdensity-luminance characteristic, FIG. 10 shows a voltage-luminancecharacteristic, FIG. 11 shows a luminance-current efficiencycharacteristic, and FIG. 12 shows an emission spectrum.

A light-emitting element is formed over a glass substrate and ITO isformed sequentially from the glass substrate side in 110 nm thick as afirst electrode 101. ITO is formed by a sputtering method and etched tohave its shape as large as 2 mm×2 mm. Next, the surface of the substrateis cleaned with porous resin (typically, made from PVA (polyvinylalcohol), nylon, or the like), heat treatment is performed at 200° C.for one hour, and UV ozone treatment is performed for 370 seconds aspretreatment for forming the light-emitting element over the firstelectrode 101.

Then, CuPc is formed as a hole-injecting layer to be 20 nm thick.Subsequently, NPB is formed as a hole-transporting layer to be 40 nmthick. Alq₃ and coumarin 6 are formed over these lamination films as alight-emitting layer to be 1:0.01 in mass ratio. The light-emittinglayer is formed to be 40 nm thick. In this embodiment, three layers ofthe hole-injecting layer, the hole-transporting layer, and thelight-emitting layer correspond to the first layer 102. Further, Alq₃and lithium are formed as a second layer 103 to be 1:0.01 in mass ratio.The second layer is formed to be 30 nm thick. Thereafter, molybdenumoxide is formed as a third layer 104 to be 110 nm thick and NPB andmolybdenum oxide are formed as a fourth layer 105 to be 1:0.25 in massratio. Note that the fourth layer 105 is formed to be 140 nm thick.Subsequently, Al is formed as a second electrode 106 in 200 nm thick tocomplete elements and lastly sealing is performed under a nitrogenatmosphere so as not to expose the elements to an atmosphere. Note thatany one of the film formations of the hole-injecting layer to the secondelectrode is performed by a vacuum vapor deposition method by resistanceheating.

FIGS. 9 to 11 show that the light-emitting element of the inventionfunctions favorably as a light-emitting element. In addition, FIG. 12shows that the light-emitting element of this embodiment emits favorablegreen light.

What is claimed is:
 1. A light-emitting device comprising: an electrode;a first layer over the electrode, the first layer comprising alight-emitting material; a second layer over and in direct contact withthe first layer, the second layer comprising an organic compound dopedwith an electron-supplying material; a transparent conductive layer overand in direct contact with the second layer; and a third layer over andin direct contact with the transparent conductive layer, the third layercomprising a carrier transporting medium, wherein each of the secondlayer and the transparent conductive layer has a single layer structure,wherein the transparent conductive layer comprises a material, thematerial being included in a bottom part of the transparent conductivelayer and an upper part of the transparent conductive layer, wherein thebottom part is in direct contact with the second layer and the upperpart is in direct contact with the third layer, and wherein thelight-emitting device does not include any layer comprising an organicmaterial over the third layer.
 2. The light-emitting device according toclaim 1, wherein the organic compound is an electron-transportingmaterial.
 3. The light-emitting device according to claim 1, wherein theelectron-supplying material is selected from an alkali metal, analkali-earth metal, and a rare-earth metal.
 4. The light-emitting deviceaccording to claim 1, wherein the carrier transporting medium is ahole-transporting medium.
 5. The light-emitting device according toclaim 1, wherein the material in the transparent conductive layer isselected from the group consisting of indium tin oxide, tin oxide, andindium oxide.
 6. An electronic device comprising the light-emittingdevice according to claim
 1. 7. A light-emitting device comprising: anelectrode; a first layer over the electrode, the first layer comprisinga light-emitting material; a second layer over and in direct contactwith the first layer, the second layer comprising a first organiccompound doped with an electron-supplying material; a transparentconductive layer over and in direct contact with the second layer; and athird layer over and in direct contact with the transparent conductivelayer, the third layer comprising a carrier transporting materialcontaining a second organic compound, wherein each of the second layerand the transparent conductive layer has a single layer structure,wherein the transparent conductive layer comprises a material, thematerial being included in a bottom part of the transparent conductivelayer and an upper part of the transparent conductive layer, wherein thebottom part is in direct contact with the second layer and the upperpart is in direct contact with the third layer, and wherein thelight-emitting device does not include any layer comprising an organicmaterial over the third layer.
 8. The light-emitting device according toclaim 7, wherein the first organic compound is an electron-transportingmaterial.
 9. The light-emitting device according to claim 7, wherein theelectron-supplying material is selected from an alkali metal, analkali-earth metal, and a rare-earth metal.
 10. The light-emittingdevice according to claim 7, wherein the carrier transporting materialis a hole-transporting material.
 11. The light-emitting device accordingto claim 7, wherein the second organic compound is doped with a metaloxide selected from molybdenum oxide, vanadium oxide, and rhenium oxide.12. The light-emitting device according to claim 7, further comprising aconductive layer over the third layer.
 13. An electronic devicecomprising the light-emitting device according to claim
 7. 14. Thelight-emitting device according to claim 7, wherein the material in thetransparent conductive layer is selected from the group consisting ofindium tin oxide, tin oxide, and indium oxide.